The proposed experiments have two major aims: (1) to describe the trigeminal primary afferent innervation of the brainstem and spinal cord, and (2) to address problems related to postnatal development and plasticity in the trigeminal system. We will employ HRP tracing and novel surgical techniques to examine the innervation of the rat brainstem and spinal cord by different trigeminal ganglion divisions as well as individual trigeminal branches. Our preliminary studies have already revealed a more complex and widespread primary afferent innervation than has heretofore been observed. Conclusions drawn from these experiments will be refined and extended by intra-axonal injection of HRP into functionally identified primary afferents. Such experiments will allow us to correlate subnuclear and laminar distributions of afferent terminals with the sensory information they convey. These studies should add significantly to our knowledge of trigeminal primary afferent organization. Because irritation or degeneration of these fibers is thought to be involved in a number of craniofacial disorders (e.g. tic douloureux), these results should also be of significant, clinical interest. The trigeminal system, particularly the innervation of the mystacial vibrissae by the infraorbital nerve, has become a leading model for examining peripheral influences upon central nervous system development. We propose experiments which should contribute the following novel information: 1) the normal organization of whisker representation in the trigeminal ganglion, 2) the infraorbital primary afferent innervation of the brainstem, 3) the way in which these develop postnatally, and 4) the manner in which they are altered by surgical manipulations used to study central trigeminal development. We will use HRP transport to define the ganglionic representations of individual whisker follicles in normal adults, neonates and adult animals subjected to either follicle cauterization or infraorbital nerve section at birth; and to examine the normal ontogeny of trigeminal brainstem innervation and the manner in which it may be altered by neonatal nerve damage. Our preliminary findings, indicating considerably greater plasticity for trigeminal primary afferents than was previously thought to be the case, have important implications for peripheral-central interaction in development.